I won't say there is a "cost" for your magical workings. What I will tell you is that in my opinion the Three-Fold Law is far too over-balanced to be considered natural. What you send out you get back. It is not three-fold, but equal. Nature always does a great job at equalizing itself. Think of it this way, if you are a jerk to everyone that is eventually going to impede your progress of meeting new people and generally being a happy individual. So if your magical workings are of poor taste and ethics then it will probably cause a negative effect for you. That'll be your "cost".

Terry Goodkind in his Sword of Truth series has a group of Wizards Rules that I have found to be very helpful and to the point in my life. One of those rules states that the greatest of harm can result from the best of intentions. How true this is. Think before you cast. We are all responsibile for what we do good or bad.

Don't worry, the cost of magic is strictly relative to the spell you perform and it varies with the kind of magic. Telepathy and astral projection, for instance, don't have a price but can effect you negatively if you don't balance yourself. Spells that cause physical effects, however, require the same amount of energy and effort as would normally, sometimes more depending on your concentration. You can exhaust yourself by doing too many spells, but you can easily replenish your energy by resting or eating. Summoning rituals have subjective costs determined by what you are summoning.

You only have to pay a high price, if you do black magick, like bringing someone back from the dead, hurting somebody and calling evil spirits/demons, other then things like that there's not much to physically/spirially pay.